Advanced, adaptive video multiplexer system

ABSTRACT

An advanced multiplexer designed and optimized for next generation on-demand video distribution is described. Features and capabilities include auto-discovery, channel-staggering and compatibility with static Virtual Channel Tables (VCTs). The multiplexer system facilitates auto-discovery by inserting identifiers into MPTSs (Multi-Program Transport Streams). These identifiers are echoed back to the multiplexer by the client set-top thereby indicating correspondence between modulators, service groups, and clients. When modulating multiple channels, FEC frames (Forward Error Correction frames) are staggered across channels to reduce correlation and clipping in the IFFT processor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 60/499,043 filed on Aug. 29, 2003, which is incorporatedherein by reference.

This application is a continuation of copending PCT Patent ApplicationNo. PCT/US 2004/028093 filed on Aug. 27, 2004, which is incorporatedherein by reference.

This application relates to copending PCT Patent Application No. PCT/US2004/028155 filed on Aug. 27, 2004, which is incorporated herein byreference.

This application relates to copending PCT Patent Application No. PCT/US2004/028031 filed on Aug. 27, 2004, which is incorporated herein byreference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to video multiplexing systems, and moreparticularly to video multiplexing systems for digital cable televisiondistribution.

BACKGROUND

Over the last several years, there has been considerable growth in theavailability of digital cable and satellite television broadcasting. Asdemand for digital programming continues to grow, cable televisionproviders are transitioning from analog cable transmission systems andconverters to mixed analog/digital and all-digital cable distributionsystems. Increasing competition from digital satellite service providershas contributed to increased demand for more and different digital cableservices including digital data services, interactive programmingservices and “on-demand” services like video-on-demand (VOD). A high-endvariant of VOD, “everything-on-demand” (EOD) offers a dedicated,full-time video and audio stream for every user. An EOD stream can beused to view time-shifted TV, movies, or other content stored by contentproviders at the headend of the network, with full VCR-like controlssuch as pause, fast forward, random access with “bookmarks”, etc.

In combination with other services like interactive programming, cableInternet services, etc., these per-user services require considerablymore infrastructure than do pure broadcast services. These newer,high-end services require a server subsystem to provide dynamicallycustomized multi-program multiplexes on a per-user basis. Clearly, thisrequires a great deal of high-speed, high-performance processing, datarouting, encoding and multiplexing hardware that would not otherwise berequired.

As demand continues to grow for these high-end, per-user services, thereis a growing need for more efficient, more cost-effective methods ofcreating large numbers of custom program multiplexes.

Television signals are commonly delivered to the home using distributionsystems based on coaxial cable, twisted-pair telephone wires, opticalfiber, or wireless terrestrial or satellite transmissions. In manycases, programming is made available at no cost to the viewer, andinstead the content providers and the content distributors areindirectly compensated based on revenues raised from advertising. Inother cases, content is made available without advertisements, and insuch cases, compensation is based on alternative sources of funding suchas donations, or subscription and pay-per-view fees that are paid by theviewer. Today, viewer fees are usually charged for premium programming,however, in the future, fees may also be charged for general programmingif such content can be delivered on-demand.

The delivery of on-demand programming is controlled by the viewer.Specifically, the viewer may be provided with the ability to select aprogram, begin playback at any time, pause and resume playback, reversethe direction of playback, speed up and slow down playback, or jump toany desired location in the program. One consequence of offeringon-demand programming is that it enables the viewer to avoid viewing theadvertisements that may have been inserted into a program, either byincreasing the playback rate or jumping further forward into theprogram. This can become problematic if relatively large numbers ofviewers are equipped with on-demand capabilities and the content ownersare deriving their compensation from revenues that originate from theadvertisers. Possible solutions to this potential problem includeimposing restrictions on the level of control that is made available tothe viewer, switching to a targeted or addressable advertising modelwhich may be better tuned to the interests of each particular viewer, orlevying a fee on the viewer in return for programming that isadvertisement free.

Any time a fee is charged to the public, either to receive premiumcontent, or to receive programming on-demand, it is important to providemechanisms to prevent unauthorized access to content delivered overpublicly accessible infrastructure. Access control is also important tolimit the viewing of content that is confidential, sensitive in nature,or deemed unsuitable for the general public for other reasons. Thesolution that has been adopted by the television industry is to deployConditional Access (CA) systems. Most CA systems use digital encryptionand are based on ciphers that encode and “randomize” the video and audiosignals. Such randomized signals can be restored only through theapplication of special keys to the cipher modules. Such keys are usuallyprotected and/or encrypted using ciphers that are even more secure thanthose applied to the signal itself. Typically, such encrypted keys areembedded into the television signal in messages known as ECMs(Entitlement Control Messages). During the presentation of a program,the keys are often changed on a regular basis and are only decodableonce a viewer has been granted access to the encrypted program or to aprogramming class that is associated with a particular encryptedprogram. Such classes of programs are known as encryption tiers.Individual viewers can be granted access to selected encryption tiersthrough the use of messages known as EMMs (Entitlement ManagementMessages). EMMs are transmitted on a relatively infrequent basis, orwhenever a change in entitlement occurs, and may be decodable only bythe intended viewer. The EMMs include the information that is needed tointerpret the ECMs corresponding to one or more encryption tiers.

Encryption equipment for television signals is deployed in cablehead-ends, satellite uplink centers, and other sites where televisionsignals are distributed. Such equipment is manufactured and maintainedby a relatively small number of vendors, and is usually based on closelyguarded proprietary technology. This protection of information helps toinsure that a system is not compromised and continues to resistunauthorized attempts to access encrypted programming. Unfortunately, bylimiting the number of vendors that have access to this market, itbecomes more difficult to introduce technological innovations and abarrier is created for new entrants seeking to enter this market withmore efficient products. For instance, hardware in a cable head-end mayinclude satellite demodulation and decryption systems, video servers,multiplexers, transcoders, encrypters, and modulators. The ability todeliver on-demand capabilities at a cost that the head-end operator canafford depends on the ability of vendors to offer such equipment atprices that are significantly lower than they are today. Unfortunately,this may not be possible if the cost of the encryption and decryptioncomponents remains high, or if these components continue to bemanufactured in low density enclosures and are not integrated with otherhead-end equipment.

SUMMARY OF THE INVENTION

The present inventive technique provides an advanced multiplexerdesigned and optimized for next generation on-demand video distribution.Features and capabilities include auto-discovery, channel-staggering andcompatibility with static Virtual Channel Tables (VCTs). The multiplexersystem facilitates auto-discovery by inserting identifiers into MPTSs(Multi-Program Transport Streams). These identifiers are echoed back tothe multiplexer by the client set-top (570′x′) thereby indicatingcorrespondence between modulators (552), service groups, and clients(570′x′). When modulating multiple channels, FEC frames (Forward ErrorCorrection frames) are staggered across channels to reduce correlationand clipping in the IFFT processor (770).

The present inventive video multiplexer system comprises a sessionmanager, a video server, a multiplexer, at least one modulator and anencrypter. The session manager establishes digital video sessions with aplurality of client devices (typically set-top boxes), and identifiesdigital video content to be provided to those client devices and howthat content is to be authorized and encrypted. Authorization (e.g., ofPay-Per-View, Subscription or On-Demand services) can be coordinatedwith a purchase server, if necessary. The digital video content isprovided by the video server and, as necessary, other sources (e.g.,satellite receiver, locally generated video stream, etc.). The videoserver is responds to session manager requests for video content andprovides such digital video content (typically from disk storage). Thevideo content typically consists of a plurality of video segments. Themultiplexer selects and combines video program content provided by thevideo server (and other sources, as applicable) into one or moremulti-channel multiplexes, transrating and/or transcoding as necessaryto match specific client video format requirements and to accommodatechannel bandwidth and/or utilization requirements. The encrypterencrypts video content according to encryption parameters associatedwith authorization information for the provided video content.Encryption channels, each encrypting according to a specific set ofencryption parameters (associated with an authorization tier), are setup as required by the session manager at the time of sessioninitialization.

According to an aspect of the invention, the multiplexer establishescorrespondence between modulators, service groups and client devices byinserting identifiers into its multi-program transport streamstransmitted to the client devices. The client devices then echo theseidentifiers back to the multiplexer via session communicationsmechanisms (via the session manager). The multiplexer compares receivedidentifiers and how they were received with how those identifiers weretransmitted to determine this correspondence. Preferably, theseidentifiers include network ID and transport stream ID parameters.

According to an aspect of the invention, each client device maintains astatic virtual channel table after session initialization. Themultiplexer accommodates these static virtual channel tables byallocating more than one virtual channel per modulator channel frequencyto a client device. This allows quick, automatic switching between videostreams by placing new video content on a different virtual channel onthe same modulator channel (physical channel) that is currently beingreceived and displayed by the client device. When a quick, seamlessswitch between video sources (segments/streams) is desired, themultiplexer causes the client device to switch virtual channels. Thisrequires little or no setup and has the added benefit that it forcesvideo resynchronization at the client device.

Ordinarily, there is little correlation across modulator channels, butit FEC (Forward Error Correction) frames are time-aligned on differentmodulator channels, there can be considerable correlation, potentiallycausing cross-channel effects and IFFT (Inverse FFT) overload. This isdescribed in greater detail hereinbelow.

According to an aspect of the invention, such cross-channel correlationis prevented by “staggering” FEC frames in time across channels suchthat they do not align. This can be accomplished by delaying eachmodulator QAM channel by a different fixed number of QAM symbols. Forexample, each QAM channel can be delayed by a multiple of 8 QAM symbols,where the multiple is determined by the number of the QAM channel.

BRIEF DESCRIPTION OF THE DRAWINGS

These and further features of the present invention will be apparentwith reference to the following description and drawing, wherein:

FIG. 1 is a block diagram of an embodiment of a network-connected videomultiplexer system employing a combination multiplexer-modulator, inaccordance with the present invention.

FIG. 2 is a block diagram of another embodiment of a network-connectedvideo multiplexer system employing separate multiplexer and modulatorfunctions, in accordance with the present invention.

FIG. 3 is a block diagram of an embodiment of a video multiplexer systemshowing a first dataflow between various system modules, in accordancewith the present invention.

FIG. 4 is a block diagram of another embodiment of a video multiplexersystem showing an alternative dataflow between various system modules,in accordance with the present invention.

FIG. 5 is a block diagram of another embodiment of a video multiplexersystem showing still another alternative dataflow between various systemmodules, in accordance with the present invention.

FIG. 6 is a block diagram of an embodiment of a transrating/transcodingvideo multiplexer system, in accordance with the present invention.

FIG. 7 is a block diagram of an integrated multiplexer-modulatorfunction, in accordance with the present invention.

FIG. 8 is a block diagram of a transrating function, in accordance withthe prior art.

FIG. 9 is a block diagram of an MPEG4 to MPEG-2 transcoding function, inaccordance with the prior art.

FIG. 10 is a block diagram showing software functions associated withthe integrated multiplexer-modulator function of FIG. 7, in accordancewith the present invention.

FIG. 11 is a block diagram of another embodiment of a video multiplexersystem employing a satellite receiver, in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

According to the present inventive technique, a video distributionsystem comprises one or more cable headends and small “edge” devices(i.e., all interconnected by a large metropolitan-area network. FIG. 1is a block diagram of one embodiment of such a system 100. In FIG. 1, a“headend” network portion of the system 100 comprises server module 110,an “encrypter” module 120, a session manager 130, and a network switch140. The network switch 140 provides connectivity between the variousheadend modules and any “edge” modules. In FIG. 1, a singlerepresentative edge module (150) is shown—an integratedmultiplexer/modulator module 150 that acts as a network edge device towhich a plurality of client devices 170A, 170B, 170C, 170D and 170Econnect via a physical distribution system 160. The physicaldistribution system 160 provides “last mile” connectivity to the clientdevices located at end user locations, and is typically provided by ahybrid fiber/coax (HFC) or similar distribution medium as shown in theFigure. In a large metropolitan system, there would typically be manysuch “edge” modules (e.g., 150) connected to the server module 110 viathe headend network switch 140. Each such edge module would in turntransmit to a similar cluster of client devices (e.g., 170′x′) via aplurality of similar “last mile” distribution networks (e.g., 160).Typically, the client devices are set-top boxes (STB's) at an end-userlocation. The server function 110 further comprises a server 110A thatprovides server resources (i.e., program content) in the form ofmulti-channel multiplexes and a server resource manager 110B foraccessing and controlling the routing of server resources within theheadend network. The encrypter function 120 provides the basis ofconditional access (CA) functionality, and further comprises anencryption “engine” 120A (encrypter) and an encrypter resource manager120B for accessing and controlling the routing of, e.g., video programcontent through the encrypter 120A. A session manager 130 acts as the“brain” of the headend network, determining what program informationwill be delivered where and how it will be encrypted. The sessionmanager 130 orchestrates the flow of video data within the headendbetween the server function 110 and the encrypter function 120, andcontrols routing of encoded/encrypted video program content to clientdevices 170′x′ via the integrated multiplexer/modulator function 150.The integrated multiplexer/modulator function 150 further comprises amultiplex resource manager 150A, a multiplexer 150B and a modulator150C. The multiplex resource manager 150A controls access to themultiplexer and determines how program content (made available to themultiplexer 150B via the network switch) is to be combined intomulti-channel multiplex streams and routed to the clients 170′x′. Amodulator 150C formats the video information for transmission over theHFC medium. Typically, the modulator 150C is a Quadrature Amplitudemodulator (QAM) operating according to a standard for digital videomodulation and transmission such as ITU-T J.83b.

In the example of FIG. 1, all program content is stored on the server110A in the headend. Typically, the server comprises one or moreinterconnected groups of video storage devices. Content from the server110A is sent to the encrypter 120A to be scrambled and thus protectedfrom unauthorized access. The multiplexer 150B then selects groups ofprograms and combines various video, audio, and data streams availableon the server 110A into one or more multiplexes (multi-program datastreams). These multiplexes are then processed by the modulator 150C anddistributed over the Hybrid-Fiber-Coax (HFC) network 160 to the clients(170′x′). Each client (or set-top) 170′x′ is then able to demodulate,decode, and display the programs on a conventional television receiver.

The session manager 130 controls the operation of the various systemmodules (server 110, encrypter 120 and integrated multiplexer/modulator150) via their respective resource managers (server resource manager110B, encrypter resource manager 120B and multiplex resource manager150A) controlling allocation of resources between the various systemmodules over the network switch. A direct communication path existsbetween the session manager and individual Resource managers linked tothe server, encrypter, and multiplexer. A less direct path existsbetween the session manager and each client, utilizing network links andmodulated upstream or downstream channels.

In the context of the present inventive technique, the process ofmultiplexing program content into multi-channel multiplexes involvestime stamping, PID (Packet ID) remapping, PMT (Program Map Table) andPAT (Program Allocation Table) creation, dejittering, and transportstream creation. Further, the creation of such multiplexes can furtherrequire transrating (rate shaping or conversion of a program stream to adifferent bit rate), statistical re-multiplexing, the handling ofvariable-bit-rate (VBR) steams, stream splicing for digital programinsertion (DPI), and transcoding between different compression methodssuch as between MPEG-4 and MPEG-2. Generally speaking, multiplexing isindependent of modulation, with little technology overlap or integrationsynergy between the two.

On the other hand, signal processing and integration synergies do existbetween multiplexing and encryption and between multiplexing and networkswitching. For these and other reasons it may be desirable to separatethe multiplexer from the modulator so that the multiplexer can berelocated from the edge device to the headend where such integrationsynergy can be better exploited. This integration synergy helps tofacilitate certain features and benefits of the present inventivetechnique that are described in greater detail hereinbelow.

FIG. 2 is a block diagram of an embodiment of a system 200 similar tothe system 100 of FIG. 1 except that a separate multiplexer module 250and modulator 252 are employed. Like the embodiment of FIG. 1, thesystem 200 of FIG. 2 includes a headend portion comprising a servermodule 210 (comprising a server 210A and a server resource manager210B), an “encrypter” module 220 (comprising an encrypter 220A and anencrypter resource manager (220B), a session manager 230, and a networkswitch 240. In addition, the multiplexer module 250 (further comprisinga multiplexer 250A and a multiplex resource manager 250B) is providedseparate from the modulator 252. Both the modulator 252 and themultiplexer module 250 are connected to other headend modules via thenetwork switch 240. The modulator 252 connects to a plurality of clients270A, 270B, 270C, 270D and 270E via a physical “last mile” network 260(shown in the Figure as a HFC network).

Dataflow between the various modules of the system 200 is shown anddescribed in greater detail hereinbelow with respect to FIG. 3.

FIG. 3 is a block diagram of an embodiment of a video multiplexer systemsimilar to those shown and described hereinabove with respect to FIGS. 1and 2, but organized to illustrate dataflow between various systemmodules. In FIG. 3, the system 300 comprises a server module 310, anencrypter module 320, a session manager module 330, a multiplexer module350 and a modulator 352. The modulator is connected to a “last mile”physical network 360 (shown in the Figure as a HFC network) to which aplurality of clients 370A, 370B, 370C, 370D, and 370E are connected.Additionally, an out-of-band modulator 354 is provided for server-clientcommunications.

The server module 310 (compare 110, FIGS. 1 and 210, FIG. 2) furthercomprises a server 310A and a server resource manager 310B. Theencrypter module 320 (compare 120, FIGS. 1 and 220, FIG. 2) furthercomprises an encrypter 320A and an encrypter resource manager 320B. Themultiplexer module 350 (compare 150, FIGS. 1 and 250, FIG. 2) furthercomprises a multiplexer 350A and a multiplex resource manager 350B.

FIG. 3 is organized to show dataflow between system modules, but thephysical system can be that shown in FIG. 1 or FIG. 2, with the networkswitch (140, FIG. 1 or 240, FIG. 2) providing communications between themodules. Since the network switch plays no active part other than datatransport, it is omitted in FIG. 3. The system modules (310, 320, 330,350, 352), the HFC network 360 and the clients 370′x′ serve the samepurposes as their counterparts in FIGS. 1 and 2. In FIG. 3, various datacommunications paths are shown. Data communications paths “S1”, “S2”,“S3” and “S4” are session communications paths controlled by the sessionmanager. Data communications paths “V1”, “V2”, “V3” and “V4” arevideo/audio communications paths over which program content is passed.Data communications path “C1” is a client communications path betweenthe server communications manager 350B and the various clients 370′x′.Data communications path “SC1” is composite of session communications S4and client communications C1 directed through the out-of-band modulator354 over the HFC network 360 for communications with the clients 370′x′.

Assuming a multiplexer system with physical connectivity similar to thatshown in FIG. 2, communications paths S1, S2, S3, S4, C1, V1, V2 and V3would all occur via the network switch (240, FIG. 2). Communicationspaths SC1 and V4 occur directly between the modulators 354 and 352(252), respectively and the HFC network 360 (260). Alternatively,communications path C1 could be routed via path S4 between the out ofband modulator 354 and the session manager 330 and path S1 between thesession manager 330 and the server resource manager 310B.

The session manager 330 (compare 130, 230) communicates with the serverresource manager 310B via communication path S1, with the encrypterresource manager 320B via communications path S2, with the multiplexresource manager 350B via communications path S3 and with theout-of-band modulator 354 via path S4. The out-of-band modulator 354provides data connectivity between the session manager 330, the serverresource manager 310B and the clients 370′x′ via the physical “lastmile” network 360, with communications “subsessions” on communicationspath S4 being set up with the various clients 370′x′ as required. Thesession manager, based upon user demand, directs video, audio and/ormultimedia program information from the video server 310A to theencrypter 320A via communications path “V1”, from the encrypter 320A tothe multiplexer 350A via communications path “V2”, from the multiplexer350A to the modulator 352 via communications path “V3” and from themodulator 352 to the clients 370′x′ over the HFC network 360 viacommunications path “V4”. The clients 370′x′ communicate with thesession manager 330 over communications paths “SC1” and “S4”. Theservers 370′x′ communicate with the server resource manager 310B overcommunications paths “SC1” and “C1”.

As described hereinabove, the present inventive technique can bepracticed using the networked components and interfaces as shown anddescribed hereinabove with respect to FIG. 1 and FIG. 2. Even assumingfixed physical network connectivity, additional benefit can be derivedintroducing changes to the flow of the video traffic within themultiplexer system. By way of example, the multiplexing process (150B,250A, 350A) can be coupled to and coordinated with the encryptionprocess (120A, 220A, 320A) via the session manager (130, 230, 330) andthe network switch (140, 240). FIG. 4 illustrates the dataflow for thisvariation.

FIG. 4 is a block diagram of another embodiment of a video multiplexersystem 400 similar to the video multiplexer system 300 of FIG. 3, butshowing an alternative dataflow topology between various system modules.

Like the system 300 of FIG. 3, the system 400 of FIG. 4 embodiescomparable elements: a server module 410 (compare 310) comprising aserver 410A and a server resource manager 410B; an encrypter module 420(compare 320) comprising an encrypter 420A and an encrypter resourcemanager 420B; a session manage 430 (compare 330); a multiplexer module450 (compare 350) comprising a multiplexer 450A and a multiplexerresource manager 450B; a modulator 452 (compare 352) and an out-of-bandmodulator 454 (compare 354). As in the system 300 of FIG. 3, theelements of the system 400 are interconnected by any suitable means,such as in a locally networked configuration as shown in FIGS. 1 and 2.The modulator 452 and out-of-band modulator 454 connect to a “last mile”network 460 (compare 160, 260, 360) shown in the Figure as an HFCnetwork. A plurality of clients 470A, 470B, 470C, 470D and 470E (compare170A-E, 270A-E, 370A-E) connect to the “last mile” network 460 andcommunicate via the modulator 452 and out-of-band modulator 454.

In FIG. 4, various data communications paths are shown. As before, datacommunications paths “S1”, “S2”, “S3” and “S4” are sessioncommunications paths controlled by the session manager; datacommunications paths “V1”, “V2”, “V3” and “V4” are video/audiocommunications paths over which program content is passed; datacommunications path “C1” is a client communications path between theserver resource manager 450B and the various clients 470′x′; and datacommunications path “SC1” is a composite of session communications S4and client communications C1 directed through the out-of-band modulator454 over the HFC network 460 for communications with the clients 470′x′.

The communications dataflow in FIG. 4 differs only in that the order ofthe multiplexer module 450 and encrypter module 420 is reversed in thevideo path. In the dataflow topology shown in FIG. 4, communication pathV1 goes from the server 410A to the multiplexer 450A, the communicationpath V3 goes from the multiplexer 450A to the encrypter 420A and thecommunication path V2 goes from the encrypter 420A to the modulator 452.

In this case, single-program transport streams (SPTSs) first exit theserver 410A to the multiplexer 450 where, if they are not already VBR(variable bit rate) streams, they can be transrated into VBR streams forbetter efficiency, and statistically re-multiplexed into a constant bitrate multi-program transport stream (MPTS) whose bit rate matches thatof the modulator 452. The MPTS is then encrypted and is transported(over the network via communication path V2) to the modulator where itis transmitted to the clients 470′x′ via the HFC network 460.

Additional multiplexing advantages can be realized if the encrypter islogically combined with the multiplexer as shown in FIG. 5. Thismodification enables the use of a multiplexed encryption scheme, wherethe multiplexer is able to select which packets are to be encrypted andis able to control how these encrypted packets are sequenced into theoutgoing streams. The benefits include much better utilization ofavailable encryption resources and the ability to eliminate thelatencies associated with the authorization of new on-demand sessions.

FIG. 5 is a block diagram of video multiplexer system 500 showing thisalternative dataflow effectively combining the functions of multiplexerand encrypter.

Like the system 400 of FIG. 4, the system 500 of FIG. 5 embodiescomparable elements: a server module 510 (compare 410) comprising aserver 510A and a server resource manager 510B; an encrypter module 520(compare 420) comprising an encrypter 520A and an encrypter resourcemanager 520B; a session manage 530 (compare 430); a multiplexer module550 (compare 450) comprising a multiplexer 550A and a multiplexerresource manager 550B; a modulator 552 (compare 452) and an out-of-bandmodulator 554 (compare 454). As in the systems shown and describedhereinabove with respect to FIGS. 3 and 4, the elements of the system500 are interconnected by any suitable means, such as in a locallynetworked configuration as shown in FIGS. 1 and 2. The modulator 552 andout-of-band modulator 554 connect to a “last mile” network 560 (compare160, 260, 360, 460) shown in the Figure as an HFC network. A pluralityof clients 570A, 570B, 570C, 570D and 570E (compare 170A-E, 270A-E,370A-E, 470A-E) connect to the “last mile” network 560 and communicatevia the modulator 552 and out-of-band modulator 554.

In FIG. 5, various data communications paths are shown. As before, datacommunications paths “S1”, “S2”, “S3” and “S4” are sessioncommunications paths controlled by the session manager; datacommunications paths “V1”, “V2”, “V3” and “V4” are video/audiocommunications paths over which program content is passed; datacommunications path “C1” is a client communications path between theserver resource manager 510B and the various clients 570′x′; and datacommunications path “SC1” is a composite of session communications S4and client communications C1 directed through the out-of-band modulator554 over the HFC network 560 for communications with the clients 570′x′.

The communications dataflow in FIG. 5 is very similar to that shown inFIG. 4, except that video communication with the encrypter 520A occursexclusively with multiplexer 550A. The communication path “V2” isbi-directional and is the only video communication path to the encrypter520. The multiplexer 550A transmits video data to the modulator 552 overthe communication path V3.

Having the functions of multiplexing put before the encryption processor coupled with the encryption process allows on-demand video networkfunctions to transrate and transcode program transport streams. Thisenables transport and delivery of VBR streams thereby utilizing serverdisk space, transport network bandwidth, and “last mile” HFCtransmission bandwidth much more efficiently. The process of switchingstreams over the network entails identifying streams, making decision onwhere they go next and with what other streams, and performing QoS(Quality of Service) processing. Those of ordinary skill in the art willimmediately appreciate and understand that these switching functions aresimilar to and related to the known processes of MPEG multiplexing whichalso involve identifying streams, labeling them, combining them withother streams, and performing QoS processing such as transrating.

By decoupling multiplexing (and the multiplex-related function oftransrating) from modulation and collocating the multiplexer (logically)with the encryption function (e.g., headend or edge), higher networkefficiency is realized because the ability to transport VBR streamsthroughout the network (as opposed to constant bit rate or CBR streams)provides 40% or more savings in disk storage space, network transportbandwidth, and last-mile HFC transmission bandwidth. On today's currenttechnological course, advancements in signal processing architectures,and system implementations can be expected to dramatically reduce thecost embedding transrating and transcoding into the multiplexerfunction. The resultant savings in disk space, transport bandwidth andHFC transmission bandwidth will then more than mitigate the cost ofadding those transcoding and transrating capabilities.

Because of these synergies and similarities in processing functions,having multiplexing near and/or combined with encryption (logically vianetworking and/or physically via integration) can result in the need forless hardware, lower stream processing latencies, and increased systemthroughput. At the same time, having a stand-alone multiplexer allowsflexibility in where multiplexing goes in a cable system and leaves thatdecision to the cable operator based upon specific system needs andsystem topologies.

The present inventive technique provides a basis for delivery ofOn-Demand Video services. It is capable of co-existing with andproviding added efficiencies to digital broadcast video distributionsystems, regardless of whether the services originate from real-timeencoders, satellite receivers, video servers, or other sources.Broadcast services can also use the transrating, transcoding,statistical multiplexing, splicing, and encryption capabilities of theinventive multiplexer for next generation on-demand video systems tosave rack space and benefit form the much lower cost per stream ofequipment designed and purchased for the on-demand video network.

Those of ordinary skill in the art will understand that the multiplexersystem of the present invention is capable of supporting a variety ofVideo On Demand (VOD) services including Movies on Demand, Subscriptionvideo on Demand (SVOD), Free Video on Demand, and other VOD-relatedservices at different bit rates and improves the efficiency,flexibility, scalability, cost and performance to provide theseservices. The flexibility of the present invention with respect totransrating, transcoding, multiplexing and encryption greatlyfacilitates such On-Demand features.

Those of ordinary skill in the art will also understand that themultiplexer system of the present invention can support HDTV(High-Definition Television) On Demand Services and improves scalabilityand flexibility with respect to delivery of HDTV On Demand Services.While modulators and upconverter are not concerned with the meaning ofthe transport streams they operate upon, multiplexer systems are tightlyconnected to the format of their transport streams. By separatingmultiplexer and modulator functionality, the present inventive techniquesimplifies deployment of HDTV and mixed HDTV/SDTV (Standard DefinitionTelevision) services by allowing multiplexers to combine both SDTV andHDTV streams into common MPTS signals.

The multiplexer system of the present invention further supports andenables interactive Digital Program Insertion (DPI) services (e.g., forinserting local advertising into a network broadcast stream) innarrowcast and on-demand video streams. Generally speaking, DPI eitherrequires pre-conditioning of streams at splice points ahead of time orrequires transrating in real time at the time of the splice.Pre-conditioning requires pre-planning and limits flexibility inselecting the points at which one can insert an advertisement.Transrating is the preferred method because it provides more flexibilityand cleaner splices. Due to the flexibility of the present inventivetechnique with respect to transrating and multiplexing, DPI is easilyaccommodated.

The present inventive technique also facilitates such features asnetworked Personal Video Recording (PVR). In a networked PVR system, thevast majority of programming typically originates from digital satellitebroadcast sources directly from the cable networks or from other digitalsatellite program sources such as HITS (an acronym for “Headend In TheSky”). These digital satellite sources are typically statisticallymultiplexed, and their video multiplex streams are in VBR form. Byproviding on-demand video networking applications with an operationallysmooth and resource-efficient way to store, transport, and deliver theseVBR streams in their efficient native VBR form, the video multiplexersystem of the present invention readily accommodates the addition of PVRservices. The present inventive techniques multiplexing capabilitieswith respect to transrating and statistical re-multiplexing are highlyadvantageous in such applications.

As with networked PVR services, the vast majority of video sources forswitched broadcast originate from digital satellite broadcast sourcesdirectly from the cable networks or from sources such as HITS. Aspreviously stated, these digital satellite sources are typicallystatistically multiplexed, and the constituent video streams in themultiplex are in VBR form. The same capabilities of the presentinventive technique that make it well suited to networked PVR servicesalso make it well suited to delivery of Switched Broadcast VideoServices.

Those of ordinary skill in the art will immediately understand that theflexibility of the present inventive video multiplexer system permitsmultiplexing functions to be shared between digital set-top box videoservices and PC streaming media services, thus, providing acost-effective platform for delivery of data network services alongsidevideo services, therefore supporting and improving the costeffectiveness of providing streaming media PC services.

MPEG-4/Part 10 (H.264) encoders and set top boxes will soon becomeavailable and it is therefore highly desirable to accommodate this andother emerging standards. The present inventive multiplexing techniqueis specifically designed to work directly with transcoders andtransraters, and is therefore ideally suited to the addition of newvideo/audio/data formats. By adding transcoding capability betweenMPEG4/Part 10 and MPEG-2, cable operators can smoothly migrate inMPEG-4/Part 10 set top boxes while continuing to use legacy MPEG-2 settop boxes in the same cable plant, in the same nodes, and in the sameservice groups. In an on-demand service environment, every set top boxhas its own point-to-point connection to the source in the headend. Ifthat source (whether it is a satellite feed, real-time encoder, or videoserver) is in MPEG-4/Part 10 format, the multiplexer can, on astream-by-stream basis, transcode to MPEG-2 to feed legacy MPEG-2On-Demand clients or keep the stream in MPEG4/Part 10 format to feed newMPEG-4/Part 10 On-Demand clients. By using MPEG4/Part 10 for satellitefeeds, real-time encoding, and/or storage on video servers, cableoperators can achieve a 2-3 times gain in the number of streams that canbe transmitted in a satellite transponder, stored on video servers,transported over the network, and sent down a QAM channel.

The present inventive multiplexing technique has a significant advantageover prior-art techniques in that it eliminates delay associated withsession authorization. In addition, the present inventive videomultiplexer can also play an important role in reducing latenciesassociated with client requests for VCR-like “trick modes” such aspause, changes in playback speed or direction, or random accesses,either to a different program or to a different location in the programthat is currently playing. The implementation of these features at themultiplexer requires a slight modification to the way that video isstreamed from the server. This latency reduction is described in greaterdetail hereinbelow.

FIG. 6 is a block diagram of an embodiment of a multiplexer module 600,in accordance with the present invention. The multiplexer module 600comprises an integrated video multiplexer function 610, Operating System(OS) RAM 620, Packet RAM 630, an Upconversion function 640, interleaveRAM 650, and a plurality of transrate/transcode modules 660A, 660B . . .660 n, each transrate/transcode module having an associated video RAM670A, 670B, . . . 670 n. The multiplexer function embodies a CPU (andassociated software), an integrated multiplexer function (MUX), anetwork interface (I/O—Gigabit or 10 Gigabit Ethernet), scrambling andmodulation. The multiplexer module 600 is representative of themultiplexer modules (e.g., 150, 250, 350, 450, 550) shown and describedhereinabove with respect to the video multiplexer systems (100, 200,300, 400, 500) of FIGS. 1-5. Within the context of the multiplexersystems shown and described hereinabove, the multiplexer modulecommunicates with a session manager and receives program streams from aserver module via a network interface. This network interface is shownin FIG. 6 as a Gigabit or 10 Gigabit Ethernet link to an externalNetwork Switch (e.g., 140, 240). The multiplexer module 600 also managesthe encryption process. Selected packets of each program are identifiedby the multiplexer function 610 for encryption and are sent to anexternal encrypter module (e.g., 120, 220, 320, 420, 520) via theGigabit Ethernet link and are subsequently returned to the multiplexerfunction 610 via the same link once they have been encrypted.Alternatively, the packets could be encrypted using local resources. Inone such alternative embodiment, the encryption keys could be receivedfrom an external encryption resource manager such as 120B, 220B, 320B,420B, and 520B and the packets could then be encrypted locally using ascrambler included in multiplexer module 610. In this case, theencryption keys would also be provided to the multiplexer in anencrypted format (i.e. as ECM packets) and these packets could then beconveyed to the receiving devices by including them in the output datastream. In a second alternative embodiment, the multiplexer couldinclude additional resources in order to generate and encrypt both thedata packets and the encryption keys. In this case, an externalencrypter (120, 220, 320, 420, 520) would not be needed at all.

In the embodiment of FIG. 6, the modulator and multiplexer are combinedinto a single integrated multiplexer function. The output of theintegrated multiplexer function 610 is a baseband, modulated multiplexstream which is presented to the upconverter function 640 forupconversion to produce an RF output for direct transmission via a “lastmile” physical transmission medium such as an HFC network.

In the alternative, the modulator function could be physically separatedfrom the multiplexer function 610 and multiplexer module 600 (as shownin FIG. 2), in which case, the output of the multiplexer function 610would be a multiplex stream that would be sent to an external modulator,preferably via the Gigabit Ethernet link and external network switch.

The integrated multiplexer function 610 also manages external resources(the transcoder/transrater functions 660′x′ and their associated videoRAM 670) to perform transrating and transcoding functions. Video RandomAccess Memory (Video RAM) 670′x′ is utilized by the transrating andtranscoding processes to store video images during conversion(transrating/transcoding). The integrated modulator portion of theintegrated multiplexer function 610 uses interleave RAM 650 to storesymbols during the interleaving step of the modulation process. The I/Oportion of the integrated multiplexer function 610 uses Packet RAM tobuffer MPEG packets, and the CPU portion of the integrated multiplexerfunction uses OS RAM 620 to store instruction and data words for bothoperating system software and application software associated therewith.

FIG. 7 is a block diagram of hardware components of an integratedmultiplexer-modulator function 700 of the type described hereinabovewith respect to FIG. 6. These hardware components are operated under thesoftware control of a processing element (i.e., CPU) included in MUXmodule 720. These components include OS RAM memory block 710A (compare620), packet RAM 710B (compare 630) and interleaver ram 710C (compare650). These random access memory devices can be implemented using commondual data rate (DDR) or single data rate (SDR) synchronous dynamic RAMdevices (SDRAM). Also included in the integrated multiplexer-modulatorfunction 700 are I/O function 730, a SERDES (Serializer/Deserializer)function 740, a “scrambler” function 750, a modulator function 760, anIFFT function block 770 and a filter function block 780.

The SERDES (Serializer/Deserializer) function 740 provides an interfaceto a high-speed network communications link, shown in the Figure as a“Gigabit Ethernet” link. The SERDES function serializes and transmitsdata packets received from the MUX 720 over the network communicationslink. It also deserializes data packets received over the networkcommunications link and delivers them to the MUX 720. These data packetscan include session level communications (exchanged with theaforementioned CPU) and video stream data to be multiplexed, forwardedfor subsequent encryption and/or modulated for transmission.

The MUX hardware 720 is essentially a selector switch that determineshow data packets will be routed, and sequences and edits these packetsas needed for seamless operation. This is done under the control of MUXsoftware running on the aforementioned CPU. The I/O module 730 assiststhe MUX 720 in routing data packets by acting as a bridge between theSERDES Gigabit Ethernet interface 740, the modulator 760, the Muxsoftware running on the CPU, MPEG packet storage in RAM block 710B, andexternal transrater/transcoder modules 660A-n. Video data packets fortransmission can be received via the SERDES function from an externalserver (e.g., 110, 210 . . . ) or encrypter (e.g., 120, 220 . . . ),from external transrater/transcoder modules via the I/O module 730, orfrom the CPU (for direct insertion into the packet stream) via MUX block720A. These video data packets are routed by the multiplexer through thescrambler 750 which encodes/encrypts the video data for transmission.The scrambler 750 passes the encoded/encrypted video data on to theJ.83b compliant modulator 760. The modulator encodes the “scrambled”video data into QAM symbols, using RAM block 710C for interleaving.After QAM encoding, the IFFT function block 770 “channelizes” the datainto a composite, multi-channel baseband data stream. This isaccomplished by delivering the QAM encoded data to the IFFT function insynchronous fashion such that one channel's data symbols are presentedat each “tap” of the IFFT, with the IFFT operated at a sample rate thatprovides the desired channel spacing between taps. After IFFT encoding,each channel is transformed such that it appears within a discretechannel frequency band of a baseband signal according to the IFFT “tap”at which is was presented. A subsequent filtering block 780 providesanti-aliasing and compensation, smoothing the baseband signal forupconversion (as required) and transmission. Preferably, the modulation,IFFT and filtering are performed in accordance with the ITU-T J.83bstandard.

FIG. 8 is a block diagram of a prior-art MPEG-2 Transrater module 800.The purpose of the transrater module is to decoder an incoming MPEG-2stream and re-encode it at a different data rate. The incoming data isinitially decoded by a variable-length decoder 805 (VLD). This is thenpresented to an Inverse Quantizer (IQ) function 810 (to restorequantized/encoded coefficient data to a “full” representation thereof).Predictive data, such as motion vectors, is operated upon by aprediction function block 845 and used in conjunction with stored frameimages to construct intermediate frame difference images in image memory855. A DCT (Discrete Cosine Transform) function 850 converts imagedifference data stored in image memory 855 and converts it into DCTcoefficients. These are summed with coefficients from the inversequantizer 810 in a summing block 815 to produce coefficient data forrequantizing at a different level (to produce a different data rate) ina Quantizer block (Q) 820. Coefficient data from the quantizer block 820is variable-length encoded (VLE) in an encoder block 825 for output as atransrated MPEG-2 data stream, and is also processed by a second inversequanitzer block (IQ) 830 and differenced with the coefficient from thesumming block 815 in a differencing block 835. The coefficientdifferences are then operated upon by an IDCT (Inverse Discrete CosineTransform) block 840 to produce difference frame image in image memory855. The processing loop comprising the summing block 815, the inversequantizer 830, the differencing block 835, the IDCT 840, the predictionblock 845, the image memory 855 and the DCT 850 is used to preventdifferences between the incoming and outgoing data streams fromdiverging as the prediction errors are propagated from one frame to thenext.

The description hereinabove with respect to FIG. 8 is specific to MPEGtransrating. Those of ordinary skill in the art will immediatelyunderstand that other types of transrating are known in the art and canbe substituted. It is fully within the spirit and scope of the presentinventive technique to do so.

FIG. 9 is a block diagram of a simple prior-art transcoder function. Inthis example, a stream that was previously encoded using MPEG4, isconverted to an MPEG-2 stream. This type of conversion can be usefulduring the process of upgrading a system from MPEG-2 to a more efficientcompression standard such as MPEG4 Part 10 (H.264). If the sourcecontent is made available in the MPEG-4 format, then the transcoder inFIG. 9 could be used when the content is to be delivered to a user whichhas not yet replaced his MPEG-2 receiver with an MPEG-4 receiver.

MPEG-4 Decode module 910 receives the MPEG-4 signal and reconstructs theuncompressed video pixels. This uncompressed stream of video pixels isthen supplied to MPEG-2 encoder 920 which is adapted to receivequantization parameters from an external Control interface, and DecisionData directly from MPEG4 Decode module 910. Decision Data can includeblock encoding parameters such as prediction mode, interlace mode,motion type, and motion vectors. If this information is supplieddirectly from MPEG-4 Decode module 910, then the complexity of theMPEG-2 encoder can be significantly reduced. However, since the encodingdecisions used in MPEG4 do not precisely match the coding decisions usedin MPEG-2, the MPEG-4 Decode module 910 would typically be designed toprovide only an approximation of the best encoding decisions to be usedby the MPEG-2 encoder. These parameters can then be further optimized byMPEG-2 Encode module 920.

The description hereinabove with respect to FIG. 9 is specific to MPEG-4to MPEG-2 transcoding. Those of ordinary skill in the art willimmediately understand that other types of transcoding are known in theart and can be readily substituted. It is fully within the spirit andscope of the present inventive technique to do so.

FIG. 10 is a block diagram of multiplexer software functions performedby the multiplexer CPU (see descriptions hereinabove with respect toFIGS. 6 and 7). These functions are described in detail inaforementioned copending PCT/JUS Patent Application No. PCT/US2004/012485, incorporated herein by reference.

Classify module 1010 examines the header of each packet of the incomingtransport stream before the packet is sent to I/O module 1020 andsubsequently stored in packet memory. The location of each packet inmemory is maintained by Priority Queue module 1040. A separate queue ismaintained for each stream. The relative priority of each queue that ismaintained by Priority Queue module 1040 is determined by PacketScheduling module 1050, based on current buffer fullness levels andexternally supplied stream priority or quality settings from ExternalAPI Ctrl module 1060. The overall transport stream data rate can beregulated by sending video packets to a transrater or transcoders.Selected packets can also be selected for encryption by CA Controlmodule 1030. These selected packets are retrieved from packet memory andsent to an external encrypter. Once encrypted, these packets are againreturned to packet memory via Classify module 1010, and are used inplace of their unencrypted counterparts. If the finaltransrated/transcoded and encrypted transport stream is to be sent to amodulator on the same network as the encrypter, then the outputtransport stream can be relayed by CA Control module 1030.

In the context of the video multiplexer system described hereinabovewith respect to FIGS. 1-5, the present inventive technique does notdirectly address interfaces and communications between the clientset-top and the server (e.g., 110, 220, 320 . . . ). However, themultiplexer can play an important role in implementing low-latencyresponses to client requests. Such requests include VCR “trick mode”functions like: pause, resume from pause, a change in playback speed ordirection, a program change, or a jump to a different access pointwithin the same program. In these cases, latency can be greatly reducedby coordinating the execution of the requested playback function withthe flushing of excess transport packets from the memory buffers of themultiplexer, client, and server. This is particularly important in thecase of statistical multiplexers which tend to utilize the fullbuffering capacity of both the client and the multiplexer, in order toefficiently accommodate VBR streams. Additional buffering is also usedby the multiplexer to implement seamless splices to or from broadcaststreams and to smooth out jitter caused by delays on the network or atthe streaming server (e.g., 102, 202 . . . ).

The present inventive technique for minimizing latency at themultiplexer is now described. It is compatible with legacy andbi-directional Open Cable compliant digital set-top boxes. There is noneed to modify Virtual Channel Tables once they have been created. (Thisis discussed in greater detail hereinbelow).

In order to minimize multiplexer delays when switching between videostreams (e.g., when implementing VCR “trick modes”), specificadaptations are made to the way video data transport originating at theserver is handled. To be able to minimize latency in processing clientrequests, the multiplexer needs additional information so that it can“get ahead of” and prepare for rapid video data switching. Specifically,the multiplexer must be able to identify the beginning of new videosegments so that it may implement an immediate transition from apreceding video segment. One solution is to make the server's StreamControl Interface (CI) available to the multiplexer. In this way, themultiplexer can be made aware of VCR-like “trick mode” requests, and canbe primed to execute a transition from one video segment to the next.However, if the new video segment originates from the same sourceaddress as the previous video segment, and is assigned to the samedestination UDP port (User Datagram Port) on the multiplexer, then theprecise transition point may still be ambiguous. For this and otherreasons, an alternative solution based on in-band signaling ispreferred. In this case, the multiplexer does not require access to theStream Control Interface (CI).

To identify transition points, the server inserts identifyinginformation at the beginning of each video segment following a requestfor a change in speed or direction, a resume from pause mode, or a jumpto a new location in the same program or to a different program. Thoseof ordinary skill in the art will immediately appreciate that there aremany possible ways of providing such identifying information.Preferably, transition points are identified using signaling accordingto the DPI Cueing Message for Cable standard (ANSI/SCTE 35). Using thisstrategy, each time a new video segment is sent from a server, it ispreceded by a single cueing message packet. In the interest ofsimplicity, this packet can be standardized to a simple static format.For example, the PID (packet identifier) could be fixed, and the spliceinfo section could include a single splice_insert command with thesplice_immediate_flag set to ‘1’. When such a packet is received at themultiplexer, the multiplexer would respond by making a rapid transitionto this new video segment. The multiplexer implements this transition byfirst verifying that the new video segment begins at a valid accesspoint. If the new segment does not begin with a sequence header andI-frame, then the transition is delayed until such headers are detected.The transition is completed by flushing all buffered packets from theprevious segment and implementing a clean entrance into the new videostream. This type of transition differs from the splices anticipated bythe DPI standard in one important respect. In this case, a cued exitpoint in the first video segment is not expected. Instead, thetransition is effected as soon as the cueing message packet is detectedin the new video segment. For this reason, the cueing message packetshould not be identical to any of the “standard” cueing messages thatare currently specified by the DPI Cueing Message standard (ANSIISCTE35). One preferred way of accomplishing this is to set theprivate_indicator flag in the splice_info_section of the cueing messageto ‘1’ when requesting an immediate splice where there is no need forsynchronization with a marked exit point in the stream that is currentlyplaying.

Clean entrances into the new video stream (i.e., entrances synchronizedwith reference frames in the new stream so that proper display of thenew stream can be accomplished immediately) are important if thetransitions are to be visually smooth. However, it is not necessary toimplement a clean exit from the previous stream. This is because weassume that a resynchronization process is triggered at the clientset-top in response to most VCR-like “trick-mode” requests. Set-tops aredesigned to flush their video and audio buffers when switching to a newchannel frequency or when changing to a different MPEG program withinthe same transport multiplex. In order to minimize latency, the sameresponse should also be triggered each time there is a request for achange in speed or direction, a resume from pause mode, or a jump to anew location in the same program or to a different program. It isassumed herein that this response can be implemented on both legacy andOpen Cable compliant set-tops even though the virtual channel may remainunchanged. Alternatively, the client set-top can be forced to flush itsbuffers and to resynchronize to a new stream segment by forcing a switchto a different virtual channel for each “trick-mode” or similartransition. In order to maintain a static definition of the virtualchannel table, the set-top can be initialized with two virtual channelsallocated to each carrier frequency. These channels will be used foron-demand programming. For each pair of virtual channels, the carrierfrequency would be the same, but the MPEG program number would beallowed to differ. As described in greater detail hereinbelow, for eachcarrier frequency, the MPEG program numbers would be uniquely assignedamong all the set-tops in a single service group. Therefore, whenswitching from one video segment to another video segment, set-topresynchronization can be forced simply by effecting a change of virtualchannel to the other virtual channel of the predefined pair at thechannel frequency to which the set top is tuned. As “trick mode” effectsare requested and responded to (on a given cable channel), the virtualchannel would simply alternate back and forth, each time causing theset-top to resynchronize with the appropriate video stream.

Preferably, segment-to-segment splices and set-top resynchronizationsare triggered in response to all VCR-like “trick mode” requests with theexception of the pause request. When a pause request is received, theserver stops sending video and the set-top freezes the image (frame)that is currently being displayed by repeatedly displaying the sameimage frame from its internal image frame memory. When the clientrequests a resumption of normal playback mode, the server resumesplayback with a new stream segment (forcing the set-top toresynchronize) and the multiplexer would executes a fast transition tothe new stream segment. The same type of response occurs when a clientrequests a change in playback speed or direction, or a jump to someother program or program location.

The technique described above can be used for implementing “trick mode”requests and guaranteeing set-top resynchronization. Those of ordinaryskill in the art, however, will immediately recognize that there areother possible schemes for coordinating “stream switching” that mightprovide more seamless or visually continuous response at the clientset-top, although most of these schemes would require the exchange ofadditional information between the set-top and the multiplexer. Thisinformation would be needed to maintain the accuracy of the set-topbuffer fullness models that are maintained in the multiplexer and usedto regulate the rate of packet transmissions. It is for this reason thatthe technique described hereinabove (switching virtual channels andusing stream cueing messages) is preferred. However, any of thesetechniques are fully within the spirit and scope of the presentinvention.

Making fast transitions from one video segment to another video segmentrequires coordinated flushing of buffers in the set-top, multiplexer,and server. However, once the transition is initiated, it is importantto restore these buffers to normal occupancy levels as quickly aspossible in order to avoid later latency problems (e.g., skips,discontinuities, pauses, etc.). If they are not restored, multiplexingefficiency will be reduced and there will be an increased risk ofunderflow as a result of processing; delays and network jitter.

In order to restore these buffers to optimum fullness levels quicklyafter a “trick mode” stream switch (or similar function), the serverassigns high priority to each new video segment for a pre-determinedinterval. While a stream is being transmitted at high priority, it isstreamed at a bit rate faster than the normal real-time rate. Thisfaster initial burst rate should be high enough so that the multiplexercan effect a stream transition as soon as it is able to identify asuitable access point in the incoming video stream (e.g., a referenceframe). While a stream is being transferred at high priority, there islittle risk of a buffer underflow at the set-top due the multiplexerfailing to receive video data from the server at a sufficient rate.

The priority of a new video segment can be restored to its normal, lowersetting once the downstream buffers have been replenished to suitablelevels. The server can determine that it has transmitted a sufficientnumber of bytes beyond real-time video display requirements since thetime of the stream switch. This can be determined by comparing videodata “accumulation” against a fixed threshold level. Alternatively, theserver can measure video “acceleration” versus time. For example, if itis desirable to maintain a half second of program data in the combinedmultiplexer and set-top buffers, then the threshold would be met afterthe server has transmitted an amount of data corresponding to the videofor the elapsed time interval since the stream switch plus an additionalhalf second. Threshold detection does not need to be overly precise aslong as the multiplexer has sufficient buffering capacity to accommodateany small overshoots which may occur. An advantage of this buffermanagement technique is that the server does not need to interact withthe multiplexer. Instead, the threshold level is either a known constantor a parameter that is communicated during initialization or when a newsession is created.

Another alternative technique for managing buffer occupancy levels is toimplement direct flow control between the multiplexer and the server. Inthis case, the multiplexer not only regulates the release of video tothe set-top, but would also “pull” data from the server at the preciserate needed to maintain optimum buffer occupancy levels. A disadvantageof this solution is that it requires additional flow control informationpassing from the multiplexer to the server, thereby presenting anadditional computing load and additional network data traffic. On theother hand, an advantage is that the same flow control information thatis useful for regulating buffer levels in the multiplexer can also beused to prevent buffer overflows in the port buffers of the networkswitch. This can become an important consideration when using unreliableprotocols such as UDP, and when stream transmission rates are not wellbounded. Short-term peaks in the video bit rate of one or more streamsor even simple network jitter could conceivably result in the loss ofpacket data by the network switch. A direct flow-control scheme managedby the multiplexer could eliminate this risk entirely. However, such anend-to-end method of flow control does not scale well in large networksconsisting of multiple switches or interfaces to other transport layers.In such cases, the path from the server to the multiplexer may beunknown or include too many switch buffers to be successfully managedfrom a single end point.

A point-to-point method of flow control, such as 802.3x, is much morecapable of scaling to larger networks. 802.3x is widely incorporated inmost of today's switches, routers, and the network transceivers used incommon computing devices. This method of flow control is effective inpreventing dropped packets in Gigabit Ethernet network switches as longas the average data rate remains below the network capacity on eachlink. However, to prevent unnecessary blocking, the network switch mustbe capable of buffering packets before they are switched to the outputports. Although 802.3x can be very effective in preventing droppedpackets in the network switch, it is not intended as a solution formanaging buffer level transients in the multiplexer.

The multiplexer can also implement transitions between stream segmentsarriving from two different IP (internet protocol) source addresses. Inthis case, the second stream is addressed to a different UDP port at themultiplexer.

A different multiplex is associated with each modulator channel even ifthe modulator and multiplexer subsystems are physically separated.During session setup, the multiplex resource manager identifies themultiplex that is best able to accommodate the new session. The IPaddress and port number corresponding to this multiplex is returned tothe session manager and subsequently relayed to the video source.

By means of the techniques described hereinabove with respect to FIGS.6, 7 and 8, the multiplexer is capable of transrating each video streamto match the data rate of the MPTS to the data rate of its assigned QAMchannel. Transrating is performed only when the data rate cannot bematched by statistical processing alone. By default, all streams aredegraded only as much as necessary to maintain uniform image qualityamong all streams in the multiplex. Alternatively, different prioritiescan be assigned to individual video streams, or the peak rates can bebounded on a stream-by-stream basis. Transrating is particularlyeffective when applied to CBR video streams where the aggregate inputdata rate exceeds the rate of the QAM channel. A conversion fromconstant bit rate to variable bit rate at constant image quality resultsin significant data rate reduction with minimal image impairment.

General-purpose data streams can be included in each multiplex anddelivered in synchronization with the video and audio components, atleast on a best-effort basis. Synchronization tolerances and QoSconstraints can be specified during session initialization.

The multiplexer can combine broadcast and narrowcast streams into thesame multiplex. Switched broadcast mode can also be supported, as wellas seamless transitioning between broadcast and narrowcast modes.Similarly, DOCSIS (Data Over Cable Service Interface Specification)streams can also be combined into the same downstream channel. Dependingupon specific characteristics of the multiplexer implementation,external devices may be used to format the DOCSIS transport packets forinclusion into the downstream channel.

The multiplexer can support multiple video formats such as MPEG-2, MP@ML(Main Profile at Main Level) and HDTV, and can include sufficientresources to handle any practical combination of MP@ML and HDTV streamsin each modulator channel. MPEG-4/Part 10 AVC (Advanced Video Coding)may also be supported with both transrating and transcodingcapabilities. With this transcoding capability of the present inventivemultiplexer, it becomes possible to conserve storage resources on theserver by adopting the more efficient MPEG4 format for most highdefinition or standard definition content, and relying on transcoding toenable compatibility with existing MPEG-2 set-tops. This ability becomesparticularly advantageous after migrating to a predominately narrowcastformat, as it permits selective enabling of the transcoding feature on aclient by client basis. This permits the gradual migration of clients toMPEG4 set-tops and can have a very significant impact on the last-milebandwidth that available for narrowcasting.

The present inventive multiplexer and multiplex resource managersubsystem is compatible with existing headend systems. With thestandardization of interfaces and APIs (Application ProgrammingInterfaces) modules may be readily interchanged.

The present inventive multiplexing system also supports existingconditional access systems. However, to permit effective transrating andtranscoding, the video must be received at the multiplexer either in theclear, or in a format that is easily decrypted. For this reason, videois streamed directly to the multiplexer from the server as shown anddescribed hereinabove with respect to FIG. 4 and FIG. 5. The multiplexeridentifies the packets to be encrypted and sends these packets directlyto the encrypter on its own (independent of the server and/or sessionmanager). The video data path associated with this mode of operation isshown and described hereinabove with respect to FIG. 5. The multiplexerand encrypter subsystems can be located either the headend or at thenetwork “edge” (i.e., at the point of modulation).

Critical packet selection using techniques similar to those developedfor Sony Passage can be applied. The present inventive multiplexer canimprove the efficiency of a central encrypter by combining criticalpackets from different streams into a single stream addressed to asingle encryption channel of a central encrypter. The encrypted packetsare then returned to the multiplexer where they are demultiplexed andresequenced into their original streams. EMMs (Encryption ManagementMessages) and ECMs (Encryption Control Messages) can be replicated asnecessary, and properly sequenced with the encrypted program packets.

Transrating and transcoding are incompatible with pre-encryption unlessthe scrambling algorithm is open and decryption routines are implementedin the multiplexer. Transrating, transcoding, and all other multiplexingfunctions are compatible with session based encryption.

In order to achieve maximum benefit of the capabilities of the presentinventive video multiplexer system in systems that employ encryptedconditional access (CA), encryption resources should be managed by themultiplex resource manager. The primary benefit is that most sessionscan be initialized without the need for allocating additional encryptionresources. Accordingly, the session manager requests resources from themultiplex resource manager before requesting additional resources fromthe encryption resource manager. Included in the session manager'srequest to the multiplexer are parameters that identify the client andthe authorization tier associated with the requested program. In mostcases, the multiplex resource manager will be able to accommodate therequest using encryption channels that have already been created, whilemaintaining maximum protection against unauthorized access. However, insome cases, the acceptance of a new request will require either theallocation of a new encryption channel or a reduction in encryptionsecurity. For example, encryption security would be degraded if two ormore clients within the same service group shared the same encryptionchannel (and therefore the same encryption keys) while viewing contentcorresponding to different authorization tiers. This condition may notbe acceptable even though the set-tops would not be provided with thevirtual channel settings needed to receive programming that is intendedfor other On-Demand clients. Encryption security would also be degradedif the critical packet ratio was reduced below a previously-determinedsafe threshold in order to accommodate an increasing number of clientssharing the resources of a single encryption channel.

When the multiplex resource manager determines that the acceptance of anew request will cause a reduction in encryption security, it respondsto the session manager with a request for the allocation of a newencryption channel. Upon receiving such a response, the session managerthen requests such resources directly from the encryption resourcemanager using interface S2 (See FIG. 5). However, if additionalencryption resources are unavailable, the session manager can eitherdeny the client's request, or allow the session to proceed with areduced level of security. The request for a new encryption channel canoccur asynchronously and without incurring a latency penalty. This isdescribed in more detail in the following session initializationexample.

The initialization management process that is presented below includesadditional details for the interactions with the multiplexer andencrypter resource managers. The system topology of FIG. 5 is assumed,and references are made to data communications paths shown in FIG. 5. Inthis particular example, the session manager always maintains a recordof the next available encryption channel. The multiplex resource managerclaims this channel only when needed to fulfill the next request.

Step 1: The client sends a session setup request message to the sessionmanager using an upstream channel within the HFC Network and InterfacesSC1 and S4. The message includes the client ID and the asset IDcorresponding to the requested program.

Step 2: The session manager (or an external purchase server) checks ifthe client is authorized to view the requested asset by comparing theclient's entitlement status with an authorization tier ID associatedwith the requested asset. If the session is authorized, go to Step 3,otherwise the session manager sends a session denial message to theclient.

Step 3: The session manager provides the client ID, authorization tierID, and the IP address/UDP destination port corresponding to the nextavailable encryption channel, and requests multiplexer resources for thesession from the multiplex resource manager (using interface S3—seeFIGS. 1-5). The multiplex resource manager replies with the virtualchannel information (tuning frequency and MPEG Program ID) for theclient. If a session is already active for this client, then thisinformation will not change unless the multiplexer chooses to rebalancetraffic over the modulator network. The multiplex resource manager isalso capable of working with static virtual channel maps for eachclient, thereby avoiding the need for constant reprogramming of theclient's receiver. (This is discussed in greater detail hereinbelow).

The response message from the multiplex resource manager to the sessionmanager also includes the multiplexer IP address/UDP destination port tobe conveyed to the server, and an acknowledgement if the new encryptionchannel was accepted. If accepted, then an entitlement ID will beincluded in the reply and the multiplexer will utilize the newencryption channel by sending selected packets to the encrypter IPaddress/UDP port that was specified in the request. The multiplexresource manager will not accept the new encryption channel if therequest can be accommodated using the encryption resources that havealready been allocated. If the encryption channel is not accepted, thengo to step 5.

Step 4: The session manager provides the entitlement ID to theencryption resource manager. This entitlement ID is associated with themost recently requested encryption channel. The encryption resourcemanager ensures that multiple encryption channels will share the sameentitlement keys if they are assigned the same entitlement ID. Theencryption resource manager responds with the EMM for the precedingencryption channel and the IP address/UDP destination port for the nextavailable encryption channel. During operation, the Encryption Enginereturns all encrypted packets to the IP address/UDP port correspondingto the originating source.

Step 5: The session manager provides the asset ID and the multiplexer IPaddress/UDP destination port to the server resource manager (S1).

Step 6: The session manager sends the session setup confirm message tothe client using S4 and SC1. Alternatively, the confirmation messagecould be relayed to the client by the multiplexer using downstreamchannels (V3 and V4) within the HFC network. The confirmation messageincludes the virtual channel information (tuning frequency and MPEGProgram ID) from step 3. In addition, if step 4 was not omitted, thenthe EMM is also included in this message.

The multiplex resource manager handles session tear-down automatically.Typically, a new session request will utilize the same multiplexer,modulator, and encrypter resources as a preceding session, and thereforethe number of session set-up and tear-down requests is minimized. Ifthere is a change in the assignment of multiplexer or modulatorresources, then this will be handled automatically by the multiplexresource manager and will not require communication with other resourcemanagers. If a client chooses to view a different asset while thecurrent session is still active, then the client merely needs to repeatstep 1. Alternatively, if a client chooses to terminate an existingsession and does not desire a new session, then the multiplex resourcemanager is notified. In some cases, this will cause selected resourcesto be released so that they can be reassigned to accommodate otherrequests.

Occasionally, the multiplex resource manager will determine that aparticular encryption channel is no longer needed. In this case, itsends a message to the session manager using S3, which the sessionmanager forwards to the encryption resource manager using S2. One way toidentify the encryption channel that is being released is to referencethe IP address/UDP port that was assigned for receiving incoming packetsarriving at the encryption engine.

The present inventive multiplexer and encryption subsystem architectureoffers several performance advantages. The number of video and audiostreams that can be accommodated by the encryption engine (encrypter) isincreased by a factor of at least 10 over traditional, lessresource-efficient approaches). In addition, the session setup andtear-down processes completely bypass the encryption resource manager inalmost all cases. A statistical multiplexer with transratingcapabilities eliminates the risk of overflowing the QAM channel even ifthe channel is intentionally oversubscribed. For example, one couldallocate 11 or more CBR video programs, each with a data rate of 3.75Mb/s, to a single 256 QAM channel. Through the selective use oftransrating, the statistical multiplexer can convert each stream fromconstant bit rate to constant image quality, thereby achieving asignificant reduction in data rate with minimal image impairment.

The present inventive multiplexer is also able to significantly reducethe latency associated with both session setup and client requests forplayback mode modifications such as VCR-like “trick modes”.Accommodation of such modes is described hereinabove. In addition, thesession setup delays that are associated with encryption can beeliminated entirely. In most cases, the client remains in the sameencryption channel even when requesting access to programming that isassociated with a different authorization tier. In such cases, there isno need to send new EMMs or to alter the flow of ECMs to the client'sreceiver. In other cases, the multiplexer may need to transition astream from one encryption channel to another encryption channel, andthis transition may also involve a change in the entitlement status ofthe client's set-top. Even in such cases, the multiplexer can ensurethat the transition is seamless, and delays due to encryption andentitlement are avoided. The multiplexer can choose to send the initialstream of packets in the clear in order to allow time for the set-top toprocess the first ECM, as well as any changes in entitlement. Thismomentary loss of access protection at the multiplexer is not asignificant concern since the session manager (or external purchaseserver) will reject any request from a client that is not properlyauthorized, and this will cause session cancellation before any requestsare sent to either the video server or the multiplexer.

Variable bit rate (VBR) encoding offers several advantages. Theimprovement in compression efficiency is approximately 40 percent whencompared to constant bit rate (CBR) programming of the same perceivedimage quality. In such a comparison, the CBR content may deliver higherquality on average; however, this is of no practical benefit whencomparative image quality during more active scenes (i.e., when imagecompression becomes more challenging) is considered. The encoded CBRcontent will exhibit higher distortion levels at these times, and thedegradation can be very noticeable when compared to VBR content encodedat the same average bit rate.

The improved efficiency achieved by using VBR encoding has a directimpact on the cost of video storage systems, the amount of networktraffic, and the number of streams that can be accommodated over the“last mile” network. This increased capacity can also be traded forhigher quality video such as HDTV programming.

Compatibility with VBR content is also important when capturingprogramming from satellite sources. Today, most satellite programming isgenerated using statistically multiplexed VBR encoders. If a videoarchitecture does not support VBR streams, then a VBR to CBR conversionprocess is required, either by increasing the bit rate with theinsertion of null packets, or by degrading the video with theapplication of a transrating process.

Compatibility with existing satellite programming is also important forsupport of Switched Broadcast. In such cases, VBR streams areselectively removed from a broadcast multiplex, and replaced withon-demand streams. In order to avoid significant image degradation, theentire multiplex can be regenerated using transraters and a statisticalremultiplexer.

DVDs also use VBR encoding, and compatibility with DVD content may beimportant for VOD systems, since DVDs are an important source of encodedmaterial. Transrating DVD content to a given VBR rate would preservebetter image quality than transrating it to CBR at the same averagerate.

Supporting VBR programming may complicate some server implementations. Aserver that is designed to stream video at a constant rate may requiremodifications before it can stream video at the actual real-timetransmission rate. The timing information needed for propersynchronization is easily extracted from the time stamps and PCRs thatare embedded in each program; alternatively the multiplexer could pullstreams from the server using a flow control mechanism, in which casethe server would actually be simplified.

The presence of VBR streams can also complicate the provisioning ofresources. For example, the number of VBR streams that can be sourced bya server may vary from one time instant to the next. The solution tothis problem involves a good load balancing strategy applied acrossmultiple storage devices. Similarly, network congestion due to trafficbetween the server and the multiplexer also becomes more difficult topredict and to control. In this case, the simplest solution is tooverprovision the network in order to satisfy worst case conditions.This additional networking cost should be negligible if the multiplexersare co-located with the servers.

Although full VBR functionality can be supported today, it may bedifficult to justify the benefits given the current cost of transratingproducts. However, we believe that a multiplexer product can be producedwithout incurring a cost penalty for statistical transratingcapabilities. Such a product could be used advantageously even if thesource content exists in a CBR format. Through the selective use oftransrating, the multiplexer can convert each stream from constant bitrate to constant image quality, thereby achieving a significantreduction in data rate with minimal image impairment. This results inadditional bandwidth that can be used to accommodate additional CBRstreams. Alternatively, the streams could be delivered to themultiplexer at a higher CBR rate, thereby realizing an improvement invideo quality.

Encryption channel management by the multiplex resource manager enablesthe use of real-time encryption at a very low cost. Compared to thepre-encryption alternative, real-time encryption offers improvedsecurity, supports more flexible key management policies, does not bindthe content to a single CA system, can be applied to programmingreceived from satellite or real-time encoders, and does not prevent theapplication of transrating, transcoding, or seamless splicingoperations. If necessary, the multiplexer could include support for someform of content encryption during distribution from the head-end sourceto the multiplexer and real-time encryption engine. In this case, themultiplexer would first decrypt the content before performing any otherprocessing.

Low cost real-time encryption is achieved by using a critical packetselection scheme similar to the packet selection methods used in SonyPassage systems. According to Sony, studies have concluded that there isa negligible loss in access protection when the critical packetselection rate is 10% or more. The multiplexer can improve theefficiency of a central encrypter by combining critical packets fromdifferent streams into a single stream that is addressed to a singleencryption channel of the central encrypter. The encrypted packets arethen returned to the multiplexer where they are demultiplexed andresequenced into their original streams. EMMs and ECMs can be replicatedif necessary, and properly sequenced with the encrypted program packets.

One consequence of multiplexed encryption is that each of the clientswhose streams are assigned to the same encryption channel will receivethe same ECMs, and therefore each client set-top must be similarlyauthorized in order to decode them. However, by careful selection of thestreams that are assigned to each encryption channel, the multiplexresource manager can eliminate the risk of a client gaining unauthorizedaccess to the content that is intended for another client. Two types ofencryption channels can be created to deal with this potential problem.The multiplex resource manager can assign packets to the first type ofencryption channel only if the corresponding streams are all associatedwith a same authorization tier. In this case, each client has littleincentive to apply his keys to another client's stream since he wouldnot be gaining access to anything that he is not already entitled toreceive.

If a client is assigned to a single-tier encryption channel, and thisclient subsequently chooses to view a program that is associated with adifferent authorization tier, then the client must be removed from thisencryption channel and reassigned to another. In some cases, this willalso require the authorization of the client's set-top in order to gainaccess to the new authorization tier. As discussed hereinabove, thistransitioning of a client from one encryption channel to another can beimplemented seamlessly without introducing any delays due to the sessionauthorization process.

The multiplex resource manager can also create and manage a differenttype of encryption channel where each of the assigned streams istargeted to clients in physically separated service groups. In thiscase, the packets of the multiplexed and encrypted packet stream areregrouped into multi-program transport streams that are subsequentlydistributed to different service groups. This means that each clientwill have no way to apply his keys to another client's stream simplybecause these streams will not be available on the same feed. Onceassigned to this type of encryption channel, a client will not need tobe reassigned when switching from one program to another, even thoughthe corresponding authorization tiers may differ. In fact, a client mayonly need to be reassigned if part of a rebalancing operation designedto avoid the over-utilization of any single encryption channel.Rebalancing can be performed seamlessly without incurring any sort ofservice disruption.

The modularized video multiplexing systems shown and describedhereinabove in FIGS. 3, 4, and 5 may span one or more cable headends andseveral edge devices distributed over a large metropolitan-area network.Managing local-area or metropolitan-area networks introduces complexity,which must be justified by benefits. Excessive complexity is a real riskin deploying new services.

LAN bandwidth, on a per-video-stream basis, is extremely cheap comparedto other costs in the VOD chain. Simply over provisioning the LAN may bethe most straightforward and least risky course. Metro networksinvolving fiber and DWDM (Dense Wavelength Division Multiplexing) arenot quite so cheap, but if they carry CBR traffic (such as completevideo multiplexes destined for “passive” modulators at the edge), thenmanagement is as simple as adding up the total traffic in the high-levelprovisioning control plane; no QoS intelligence in the networkinfrastructure is needed for this. For VBR traffic (such as in SPTSswhen video multiplexing/transrating occurs at the edge), a fixedfractional overhead can offer acceptably low probability ofoversubscription.

Interfaces for QoS and “traffic management” are well known to those ofordinary skill in the art, both for IP networking (e.g. Diffserv(differentiated services), PHBs (per-hop behavior)) and for ATM(Asynchronous Transfer Mode). Generally speaking, the operationalcomplexity and added cost of these features (especially for ATM, whichhas essentially priced itself out of the market for all but legacytelecom applications) is unlikely to justify any savings in networkcapacity.

For “next generation” video systems, it is assumed herein that thetraditional “edge” multiplexer/modulator device has been partitionedinto distinct multiplexer and modulator components as shown in FIG. 5. Aprimary motivation for this partitioning is to facilitate the option ofrelocating the multiplexer from the network edge to the headend.However, when components of next generation video systems areco-located, it may be advantageous to offer some of these components asintegrated units, particularly if this merging of components leads tolower costs, reduced space, and lower power consumption. Once thisintegration causes an interface to become internalized, compliance withinterface specifications that would otherwise exist between separatedevices is not essential provided that external interfaces andcommunications remain unaffected.

Referring once again to FIG. 5, interface S2 between the session managerand the encryption resource manager is used to reserve new encryptionchannels or to tear down existing ones. An example of how a new channelmight be reserved was described in step 4 of the process describedhereinabove. In this example, the session manager communicates anentitlement-ID to the encryption resource manager, which then respondswith an EMM to be used for client authorization, and an IP address/UDPport to be forwarded to the multiplex resource manager. This port is thenetwork address where packets are to be sent for encryption. Theencryption resource manager should also ensure that multiple encryptionchannels will share the same EMMs and ECMs if the channels are requestedusing identical entitlement-ID parameters.

The session tear-down process was also described hereinabove. Themultiplex resource manager determines when an encryption channel is nolonger needed and communicates this information to the session manager.The session manager then forwards this message to the encryptionresource manager using S2. The tear-down message merely needs toidentify the encryption channel that is no longer needed.

The following information should be included in the session set-upmessage communicated on S3:

client ID: used by the multiplex resource manager in order to uniquelyidentify each client. A mechanism must also be provided to specify ifthe session is to be multicasted to more than one client or broadcastedto one or more service groups. For example, reserved bits of the clientID codeword may be used to signal broadcast mode. Alternatively, theclient ID could include a network mask in order to specify multiplesubnets.

Authorization Tier: this is an authorization classification that isassociated with the requested asset and is used by the multiplexresource manager when assigning streams to encryption channels based onthe single-tier policy.

IP address/UDP port (encryption): the address of the next availableencryption channel. It is to be used only if the multiplex resourcemanager cannot identify an existing encryption channel to accommodatethe request while still meeting the minimum security requirement. Acodeword may be reserved to indicate that no additional encryptionchannels are available.

QoS: specifies the channel priority to be assigned for this request. Itis used by the multiplex resource manager when selecting an MPTS toaccommodate the request and when managing the application of transratingonce the selected modulator channel becomes fully utilized. Transratingcan be controlled by including a relative stream priority ormaximum/minimum bit rates as part of the QoS specification.

Time: can be used to specify the session start time or to signal the useof DPI cueing messages.

The response to the session setup message on Interface S3 should includethe following information:

Virtual channel ID: This information may include the tuning frequencyand MPEG Program ID needed by the client's set-top to receive the MPTSand to identify the assigned MPEG program. Alternatively, the multiplexresource manager could be synchronized with the Virtual Channel Table(VCT) of each client's set-top, and in this case, only the VCT ID needsto be returned. This enables the use of static VCT tables and avoidsadditional delays due to set-top reprogramming.

IP address/UDP port (input): the address that has been assigned toreceive packets for this program. It is to be forwarded to the server.

Entitlement ID: if the offered encryption channel was accepted, thenthis parameter should be forwarded to the Encryption Resource manager.If the Encryption Resource manager subsequently determines that theentitlement ID was previously bound to one or more existing encryptionchannels, then the new channel should share the same EMM and ECMs as theprevious channel(s). Otherwise, the new channel should be associatedwith independent EMM and ECM messages. A reserved codeword can be usedto indicate that the offered encryption channel was not accepted by themultiplex resource manager. If an additional encryption channel was notavailable and therefore not offered to the multiplex resource manager,and if the request could not be accommodated while meeting the minimumsecurity requirement, then this must be signaled as well.

Although the minimum security requirement parameter should also bespecified to the multiplex resource manager, this may be done duringinitialization and need not be repeated each time a session setupmessage is exchanged.

If a session is to be terminated and if the client will no longer beactive, the server can simply stop sending packets to the identifiedinput IP address/UDP port. Alternatively, explicit session terminationmessage which references the client-id could be sent from the sessionmanager to the multiplex resource manager. Finally, it is not necessaryto terminate sessions when clients switch from one program to another.The multiplex resource manager can implement a seamless low-latencytransition when the next session set-up message is received.

Since the modulator channel is assigned by the multiplex resourcemanager, the session set-up message does not need to involve themodulator. However, if the multiplexer and modulator are separatedevices, then the multiplex resource manager must be aware of thedestination IP address and UDP port assignments corresponding to eachmodulator channel, and the correspondence between modulator channels,service groups, and clients. This information should be gathered duringthe initialization process and updated from time to time as changesoccur. This client initialization and auto-discovery process isdescribed in more detail hereinbelow.

A suitable choice for the transport interface sourced by the server(Interface VI) is raw UDP/IP over a Gigabit Ethernet physical link. Oneto seven MPEG transport packets can follow the UDP header, formatted asan SPTS according to the ISO/TEC 13818-1 MPEG transport specification.RTP headers are optional.

A suitable choice for the transport interface sourced by the encrypter(Interface V2) is raw UDP/IP over a Gigabit Ethernet physical link. Oneto seven MPEG transport packets can follow the UDP header, formatted asan SPTS according to the ISO/IEC 13818-1 MPEG transport specification.RTP headers are optional.

A suitable choice for the transport interface sourced by the multiplexer(Interface V3) is raw UDP/IP over a Gigabit Ethernet physical link. Oneto seven MPEG transport packets, formatted as an MPTS according to theISO/TEC 13818-1 MPEG transport specification, can follow the UDP header.Although RTP headers are acceptable, they are not useful to themultiplexer, since transport packets must still be checked for thepresence of PCRs (Program Clock Reference). The PCRs are necessary forindividualized synchronization with the time base that is associatedwith each MPEG program.

A suitable choice for the transport interface sourced by the modulator(Interface V4) are MPTS's over fiber or coax with 6 MHz or 8 MHz FDMchannelization. Each MPTS should be compliant with ISO/IEC 13818-1 aswell as applicable DOCSIS specifications.

The client auto-discovery process is assisted by the multiplexer, whichcan insert network-id and transport-stream-id parameters into eachmultiplex. During initialization, the multiplex resource manager learnsthe network address of each modulator channel (unless the multiplexerand modulator are integrated into the same device) and the channelfrequencies and service groups associated with each modulator channel.The multiplex resource manager must also determine which clients areincluded in each service group. All of this information can bedetermined automatically if the client's set-top can be programmed toextract the network-id and transport-stream-id parameters from thedownstream channels and echo this information back to the multiplexresource manager using an upstream channel within the HFC Network andInterfaces SC1, S4, and S3.

As described hereinabove, the multiplex resource manager is compatiblewith static Virtual Channel Table (VCT) assignments at the client. Thisavoids the need for constant reprogramming of the client's set-top andthe associated delays during session set-up. In this case, the VCTshould be constrained in order to contain at least one entry for eachmodulator frequency that is managed by the multiplex resource manager.The entry should also specify an MPEG Program ID that is not shared byany other client set-tops within the same service group when tuned tothe same frequency.

Since the present inventive multiplexer, encrypter, and modulatorsubsystems are designed and optimized for Networked PVR applications, noadditional modifications are required. Together, these subsystems arewell-suited to applications requiring a large number of simultaneoussessions, real-time encryption, minimal setup and tear-down signaling,fast transitions from one session to another session, and rapid responseto client requests for VCR-like “trick mode” functions. Switchingbetween broadcast and on-demand sessions can also be performedseamlessly as described hereinabove.

To minimize complications in server and asset management subsystems,on-demand capabilities should be disallowed while viewing broadcastcontent unless the asset is already registered and in the process ofbeing captured (i.e., recorded to disk or other storage medium). In thiscase, it is advantageous to use the captured content as the source forthe broadcast signal. Although a slight loop-through delay would beintroduced, the transitioning of one or more clients to on-demand modecould then be implemented seamlessly and access to the entire recordingcould be made available at that time.

FIG. 11 is a block diagram of a video multiplexer system 1100 similar tothat previously shown and described hereinabove with respect to FIG. 4.The video multiplexer system 1100 of FIG. 11 accommodates this schemefor a satellite receiver.

Like the system 400 of FIG. 4, the system 1100 of FIG. 11 embodiescomparable elements: a server module 1110 (compare 410) comprising aserver 1110A and a server resource manager 1110B; an encrypter module1120 (compare 420) comprising an encrypter 1120A and an encrypterresource manager 1120B; a session manager 1130 (compare 430); amultiplexer module 1150 (compare 450) comprising a multiplexer 1150A anda multiplexer resource manager 1150B; a modulator 1152 (compare 452) andan out-of-band modulator 1154 (compare 454). As in the system 400 ofFIG. 4, the elements of the system 1100 are interconnected by anysuitable means, such as in a locally networked configuration as shown inFIGS. 1 and 2. The modulator 1152 and out-of-band modulator 1154 connectto a “last mile” network 1160 (compare 160, 260, 360, etc.) shown in theFigure as an HFC network. A plurality of clients 1170A, 1170B, 1170C,1170D and 1170E (compare 170A-E, 270A-E, 370A-E, etc.) connect to the“last mile” network 1160 and communicate via the modulator 1152 andout-of-band modulator 1154.

In FIG. 11, various data communications paths are shown. As before, datacommunications paths “S1”, “S2”, “S3” and “S4” are sessioncommunications paths controlled by the session manager; datacommunications paths “V1”, “V2”, “V3” and “V4” are video/audiocommunications paths over which program content is passed; datacommunications path “C1” is a client communications path between theserver resource manager 1150B and the various clients 1170′x′; and datacommunications path “SC1” is a composite of session communications S4and client communications C1 directed through the out-of-band modulator1154 over the HFC network 1160 for communications with the clients1170′x′. In addition, the multiplexer 1150A receives video stream datafrom a satellite receiver 1160 over a data communication path “V5”.

The communications dataflow in FIG. 11 differs only from that of FIG. 4in the addition of the satellite receiver 1160 and its associated videodata path V5.

The present inventive multiplexer subsystem supports Interactive DigitalProgram Insertion. Splices can be signaled according to ANSI/SCTE 30/35DPI standards. Program exit and entrance points can be defined in bothlive and stored streams, either of which may be broadcasted ornarrowcasted from any network source address. The splices areimplemented seamlessly and accurately and can be scheduled at any time,either with or without cueing messages. Transrating eliminates the riskof exceeding modulator channel capacity, even when splicing to a higherrate stream or when the splice point has not been conditioned usingfade-to-black or other rate-reduction effects. Statistical ratemanagement policies are used to achieve the best image quality balanceamong all programs, while adhering to previously-applied QoSconstraints.

To support switched broadcast, entire broadcast multiplexes can be sentto the multiplexer without unpacking the individual programs into singleprogram transport streams. For example, the Satellite Receiver shown inFIG. 11 can provide MPTSs directly to the multiplexer using interfaceV5. Although the multiplexer can support MPTSs just as easily as SPTSs,this requires the inclusion of additional information in the sessionsetup API that is communicated using Interface S2 between the sessionmanager and the multiplex resource manager. Specifically, the sessionmanager must specify to the multiplex resource manager which MPEGprograms of the MPTS are to be forwarded to the destination, oralternatively, which programs are to be deleted. The session managermust also inform the multiplex resource manager each time a decision ismade to drop an additional program or to reinstate a program back intothe multiplex.

As programs are dropped and additional bandwidth becomes available, themultiplex resource manager can automatically introduce on-demandprogramming into the multiplex in order to better utilize the availablebandwidth. As described hereinabove, new streams can be provided to theclient with very little latency.

In providing streaming media services to PCs and other devices overDOCSIS, the video content could be sent either natively in the MPEG-TSsublayer or encapsulated in IP according to the DOCSIS MAC layer.Existing cable modems (CMs) would not bridge the native MPEG-TS packetsonto the home network, so in the interest of reverse-compatibility, theuse of IP is preferred, perhaps using well-known multicast addresses.

A converged video and DOCSIS infrastructure would eliminate thedistinction between video QAMs and DOCSIS QAMs, in which case video datacould be migrated over to an IP format on a user-by user basis. Theusers could view the content either on a PC or on a video-enabled CM (orboth). Moving away from legacy set-tops and toward simpleDOCSIS/IP-based video endpoints would result in significant costsavings. In this scenario (converged video/DOCSIS), since the contentwould already be stored at the headend in MPEG-2 or MPEG4/Part 10format, it could be sent as-is to the PC or CM; it would seem to bedisadvantageous to store additional copies in the proprietary, closedformats commonly seen on the Internet. Content selection could beaccomplished via an HTTP front-end to the headend infrastructure;playback control could again be HTTP or some other form of RPC. Headendservices could be modularized to allow this to happen cleanly, much asweb services are partitioned into HTTP-centric front ends anddatabase-centric back ends.

When processing many QAM signals in a multi-channel modulator DSPenvironment, dynamic range, crest factor, and clipping are importantconsiderations. Correlations in the data across QAM channels canintroduce stress into the IFFT processor (e.g., 770), since a nominalIFFT scaling will be optimized for near-Gaussian signals—coherent datawill cause the IFFT to overflow or clip.

Most MPEG payload data is relatively uncorrelated across channels, butif FEC (Forward Error Correction) frames are aligned across channels,the FEC headers themselves can introduce correlations. One solution isto offset the symbol streams from one another post-trellis coding bydelaying each QAM channel a different number of QAM symbols. An offsetof eight QAM symbols per channel (so that channel 0 is delayed 0 bysymbols, channel 1 is delayed by 8 symbols, channel 2 is delayed by 16symbols, etc.) is sufficient to remove all such cross-channelcorrelations.

It seems likely that the most cost-effective storage systems will beblock-based and distributed. Serving a given video stream entirely fromone server node does not allow good system-wide load balancing, and itresults in inefficient RAID (Redundant Array of Inexpensive Drives)schemes. Distributed dynamic RAID across all storage nodes offers animproved level of robustness at the lowest cost.

The “streaming” function, that is, emitting video bits at the correctaverage rate, is currently handled by the servers. It could instead behandled by the video multiplexer, since the multiplexer is the naturalaggregation point for content contributions from all storage nodes. Themultiplexer could implement the RAID scheme and interoperate with adistributed file system.

For networking, 10G (10 Gigabit Ethernet) is the clear choice for LAN(Local Area Network) and MAN (Metro Area Network) as soon as it hitscost parity with 1G (1 Gigabit Ethernet). 10G is a natural convergencepoint for Ethernet and SONET, so legacy SONET installations cantransition to the more cost-effective Ethernet. Centralizedinstallations with system radii under 50 meters or so will be able touse 10G/copper with even greater cost savings.

The last-mile network is currently a hybrid of analog and digitalsignals channelized at 6 MHz. While the channelization is an artifact ofthe legacy terrestrial FDM (Frequency Division Multiplexed) analogtelevision system, it is probably useful even in an all-digital scenarioto control receiver cost, at least until individual users requireservices at greater than 40 Mb/s.

It will soon become cost-effective to transition to an all-digital lastmile. “Micro-set-tops” or “converters” from digital to analog RF willquickly pay for themselves, given the high-value services that will usethe harvested bandwidth, and in high volume will be very cheap.Moreover, the modulators to supply the waveforms for these manyadditional digital channels can be implemented very inexpensively usingDSP techniques and high sample-rate, high-fidelity DACs. A typicalmodulator might accept a 1 OG feed and produce two RF outputs each with128 QAM channels; several such modulators could fit in a single rackunit and a −100W power budget, even with today's technology.

Finally, it seems likely that video and DOCSIS will converge. Thedownstream PHYs are identical, and there is a convergence sublayer inthe form of MPEG-TS. Current CMTSs bind downstream and upstream MACprocessing rather tightly, but the DOCSIS protocol itself doesn'tpreclude “split” implementations in which a downstream box handles allthe downstream (including video content), an upstream box handles allthe upstream, and MAC processing and the link to IP occurs in some thirdbox sitting on the network. Timing requirements could be handled byhigh-resolution timestamps appended to the MAC fragments, both up anddown; it is easy to distribute accurate time, even over wide areas.

The present inventive technique is frequently described hereinbelow inthe context of digital cable television systems. However, those ofordinary skill in the art will understand that with appropriate, minoradaptations, the invention can also be applied to other videodistribution systems such as telephone distributions networks comprisingnetwork switching gear at the central office and fiber or twisted paircabling from the central office to the home. In some cases, telephonecompanies may offer video services using similar 6 MHz or 8 MHzchannelization of the frequency spectrum, and therefore will specifymodulator devices which may be identical to the cable systemcounterparts. In other cases, the modulator will be replaced by a DSLAM(Digital Subscriber Line Access multiplexer) or some other device morecapable of maximizing the data throughput over the last mile physicallink. In such cases, it is only the modulator component of the videodistribution system that is replaced. The multiplexer, which is wherethe present inventive techniques are realized, will remain essentiallyunchanged.

Although the invention has been shown and described with respect to acertain preferred embodiment or embodiments, certain equivalentalterations and modifications will occur to others skilled in the artupon the reading and understanding of this specification and the annexeddrawings. In particular regard to the various functions performed by theabove described components (assemblies, devices, circuits, etc.) theterms (including a reference to a “means”) used to describe suchcomponents are intended to correspond, unless otherwise indicated, toany component which performs the specified function of the describedcomponent (i.e., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure which performs thefunction in the herein illustrated exemplary embodiments of theinvention. In addition, while a particular feature of the invention mayhave been disclosed with respect to only one of several embodiments,such feature may be combined with one or more features of the otherembodiments as may be desired and advantageous for any given orparticular application.

1. A video multiplexer system characterized by: a session manager forestablishing digital video sessions with client devices, for identifyingdigital video content to be provided to said client devices, and howsaid digital video content is to be encrypted; a video server,responsive to said session manager, for providing said digital videocontent, said video content being further characterized by a pluralityof video segments; a multiplexer for selecting and combining videosegments into one or more multi-channel multiplexes; at least onemodulator for encoding and transmitting MPTSs to the client devices;means for encrypting digital video content; means for establishingauthorization tiers for encrypting digital video content; wherein: themultiplexer establishes inserts identifiers into MPTSs transmitted tothe client devices; the client devices echo the received identifiersback to the multiplexer; and the multiplexer establishes correspondencebetween correspondence modulators, service groups, and client devices bycomparing the identifiers echoed by the client devices with theidentifiers inserted by the multiplexer.
 2. A video multiplexer systemaccording to claim 1, wherein: the inserted identifiers include networkID and transport stream ID parameters.
 3. A video multiplexer systemaccording to claim 1, wherein: each client device maintains a staticvirtual channel table after session initialization.
 4. A videomultiplexer system according to claim 3, wherein: the session managerestablishes more than one virtual channel per physical channelestablished with the client device.
 5. A video multiplexer systemaccording to claim 3, wherein: a client device receives a video streamon a specific virtual channel on a specific physical channel; themultiplexer effects rapid switching of the client device to a differentvideo stream by providing the different stream on a different virtualchannel on the same physical channel and causing the client device toswitch virtual channels.
 6. A video multiplexer system characterized by:a session manager for establishing digital video sessions with clientdevices for identifying digital video content to be provided to saidclient devices, and how said digital video content is to be encrypted; avideo server responsive to said session manager, for providing saiddigital video content, said video content being further characterized bya plurality of video segments; a multiplexer for selecting and combiningvideo segments into one or more multi-channel multiplexes; at least onemodulator for encoding and transmitting MPTSs to the client devices overa plurality of physical channels at a like plurality of channelfrequencies; means for encrypting digital video content; means forestablishing authorization tiers for encrypting digital video content;wherein: FEC frames are staggered across modulator physical channels toprevent cross-channel alignment thereof.
 7. A video multiplexer systemaccording to claim 6, wherein: each modulator channel is QAM encoded;and each QAM channel is delayed by a different fixed number of QAMsymbols.
 8. A video multiplexer system according to claim 7, wherein:each modulator channel is delay by a multiple of 8 QAM symbols; and themultiple is determined according to the number of the modulator channel.9. A video multiplexer system characterized by: a session manager forestablishing digital video sessions with client devices, for identifyingdigital video content to be provided to said client devices, and howsaid digital video content is to be encrypted; a video server,responsive to said session manager, for providing said digital videocontent, said video content being further characterized by a pluralityof video segments; a multiplexer for selecting and combining videosegments into one or more multi-channel multiplexes; at least onemodulator for encoding and transmitting MPTSs to the client devices overa plurality of physical channels at a like plurality of channelfrequencies; means for encrypting digital video content; means forestablishing authorization tiers for encrypting digital video content;wherein: the multiplexer is adapted to operate with staticallyprogrammed virtual channel tables in the client devices.
 10. A videomultiplexer system according to claim 9, wherein: for each channelfrequency, all client devices associated with a service group assignedto that channel frequency are initialized with a unique MPEG Program ID.